This section provides background information related to the present disclosure which is not necessarily prior art.
Based on the popularity of four-wheel drive vehicles, a number of different power transfer systems are presently available for selectively directing power (i.e. drive torque) from a powertrain to all four wheels of the vehicle. Typically, the powertrain transmits drive torque to a first or primary driveline for driving a pair of primary wheels. The power transfer system includes a power transfer device arranged to continuously or selectively transmit a portion of the drive torque to a second or secondary driveline for driving a pair of secondary wheels. In particular, some power transfer devices are equipped with a mode shift mechanism including a mode clutch that can be selectively actuated to shift the motor vehicle between a two-wheel drive mode and a four-wheel drive mode. In the two-wheel drive mode, the mode clutch is released and drive torque is only transmitted via the primary driveline to the primary wheels. In the four-wheel drive mode, the mode clutch is engaged and drive torque is additionally transmitted via the secondary driveline to the secondary wheels.
Many power transfer devices, such as transfer cases and power take-off units (PTU), are equipped with a mode shift mechanism having a sliding sleeve-type mode clutch to facilitate mode shifting when the motor vehicle is moving or at rest. Such sleeve-type mode clutches typically include a clutch hub driven by the primary driveline, a clutch gear drivingly connected to the secondary driveline, and a mode sleeve coupled for rotation with the clutch hub and which is slidably moveable thereon between first and second mode positions. In the first mode position, the mode sleeve is disengaged from coupled engagement with the clutch gear so as to establish the two-wheel drive mode. In the second mode position, the mode sleeve drivingly couples the clutch gear to the clutch hub so as to establish the four-wheel drive mode. The mode clutch may also include a synchronizer assembly that is operably disposed between the mode sleeve and the clutch gear to facilitate speed synchronization between the clutch hub and the ditch gear prior to complete movement of the mode sleeve into its second mode position.
The mode shift mechanism may further include a power-operated shift actuator for selectively moving the mode sleeve between its first and second mode positions. Such a power-operated shift actuator may include an electric motor driving a rotary-to-linear conversion device which, in turn, is configured to axially translate a shift fork for slidably translating the mode sleeve. A manually-operated mode select device (i.e., lever, push-button, toggle switch, etc.) located in the passenger compartment of the vehicle may be engaged to select between the two-wheel drive mode and the four-wheel drive mode. Upon selection of the desired drive mode, a mode signal is transmitted to a controller for controlling actuation of the electric motor to cause axial translation of the mode sleeve toward the mode position corresponding to the selected drive mode.
However, during an attempted mode shift from the two-wheel drive mode into the four-wheel drive mode, a “blocked” condition may occur between the engagement teeth on the mode sleeve and the clutch teeth on the clutch gear. Typically, a blocked condition exists when the engagement teeth on the mode sleeve are aligned with the clutch teeth on the clutch gear so as to prevent the mode sleeve from moving completely into its second mode position. When such a blocked condition exists, further axial translation of the mode sleeve toward its second mode position may only occur if the clutch gear is rotatably indexed relative to the mode sleeve to eliminate the blocked tooth condition. Accordingly, a significant axial force must be applied through the mode sleeve to the clutch gear to exert an index torque that is adequate to rotatably index the clutch gear relative to the mode sleeve.
Despite the shortcoming noted above, many conventional power transfer devices are equipped with a power-operated mode shift actuator that is only capable of outputting an axial force (generated via the electric motor and the rotary-to-linear conversion device) to the mode sleeve having a magnitude sufficient to complete the 2WD/4WD mode shift during an “unblocked” condition. To overcome blocked conditions, some conventional mode shift mechanisms may incorporate a spring-biasing arrangement configured to normally bias the mode sleeve toward its second mode position and assist in completing the 2WD/4WD mode shift once the tooth block condition has been eliminated due to rolling movement of the vehicle. To address this shortcoming, it has been proposed to increase the magnitude of the axial force transmitted by the power-operated mode shift actuator to the mode sleeve. Unfortunately, such a solution would be undesirably costly and require large mode shift components and high power actuators. Accordingly, a recognized need exists in the industry to provide a reduced effort mode shift mechanism that permits use of smaller and less expensive components and low power actuators while providing an axial force sufficient to overcome most tooth block conditions.